Presentation Information
[P2-55]An attempt to lower melting point of sintering aids for low temperature sintering of Sm2Fe17N3 magnets
*Tatsuya Shiratori1, Yuta Iida1, Teruta Inoue1, Akihide Hosokawa2, Wataru Yamaguchi2, Yusuke Hirayama2 (1. Niterra Co., Ltd. (Japan), 2. AIST (Japan))
Keywords:
Sm2Fe17N3,low temperature sintering,sintered magnets
Increasing demand for motors with the development of electrification technology has driven the use of Nd-Fe-B magnets in recent decades, and consequently the concerns about the supply chains of this type of magnets arose. Not only are heavy rare earth elements (e.g., Dy, Tb) used in Nd-Fe-B magnets, but even the main constitutive element, Nd, is also a matter of concern. Therefore, there is an urgent need to develop alternative materials for Nd-Fe-B magnets that are composed of elements inexpensive and have a stable supply. As a new class of post-Nd-Fe-B magnets, our research group are currently developing Sm2Fe17N3 sintered magnets, but the volume fraction of the main phase was around 90 % due to the difficulty of sintering. Although it is well known that Zn is a relatively good sintering aid (i.e. reacts with Sm2Fe17N3 main phase), it is also known that the remanence is seriously decreased by addition of Zn. The other single elements with low-melting point (e.g. In, Pb, Sn etc.) do not contribute to the enhancement of the magnetic performance of sintered magnet either. To overcome this issue, we surveyed binary alloys with low-melting points to achieve high density sintered Sm2Fe17N3 magnets. Recently, we found that Ba-Cu alloy can increases the volume fraction of the main phase as well as the remanence. The achieved remanence was 1.13 T ((BH)max=195 kJ/m3), which was not achievable with the case of Zn [1, 2]. However, further improvement of (BH)max is necessary for practical applications and for that, not only remanence but also coercivity must be significantly improved. It is established reducing particle size is beneficial to improve the coercivity and (BH)max but it is also known that reducing the particle size excessively is sometimes harmful [3, 4]. Moreover, it is getting clear that the sintering temperatures we have adopted (450 ~ 500 ℃) sometimes result in coercivity degradation upon sintering even under low oxygen atmosphere. It is considered that this degradation is because of α-Fe formation originated from the oxides that necessarily forms during the excessive pulverization process. An approach we came up with here is to develop sintering aids lower melting-points, say 300 ℃. Since the melting point of Ba-Cu that we reported is 450 ℃, sintering below this temperature has been difficult. Therefore, a composition exploration was conducted to lower the melting point of Ba based additive alloys further.
Experimental
The melting point measurements of the prepared additive alloys were conducted by DSC (Differential Scanning Calorimetry). Next, the additive alloys were added to the low-oxygen pulverized Sm2Fe17N3 powder and sintering was performed under various temperature conditions. The evaluation of sinterability in sintered bodies was conducted using the Archimedes method and electron microscopy. The presence of α-Fe phase in the obtained sintered body was evaluated by XRD pattern, and the evaluation of magnetic properties was conducted by VSM. Furthermore, the entire process from sintering aid addition step to sintering was conducted in a low-oxygen atmosphere.
Results
As a result of the exploration of low-melting point compositions of Ba based alloys, we found the alloy with a melting point of 270 ℃ (< 300 ℃) and achieved sintered bodies with a relative density of over 85 % even at sintering temperatures of below 350 ℃. According to the XRD profile, lowering the sintering temperature suppressed the precipitation of the α-Fe phase, and the coercivity did not decrease.
References
[1] R. Matsunami, M. Matsuura, N. Tezuka, S. Sugimoto, J. Magn. Soc. Jpn., 44, 64-69 (2020).
[2] Niterra Co., Ltd., AIST, press release “Development of high-density technology for Sm2Fe17N3 permanent magnet” (10.9.2024).
[3] Y. Hirayama, A. K. Panda, T. Ohkubo, K. Hono, Scr. Mater., 120, 27-30 (2016).
[4] A. Hosokawa, W. Yamaguchi, K-Suzuki, K. Takagi, J. Alloys Compds., 869, 159288 (2021).
Experimental
The melting point measurements of the prepared additive alloys were conducted by DSC (Differential Scanning Calorimetry). Next, the additive alloys were added to the low-oxygen pulverized Sm2Fe17N3 powder and sintering was performed under various temperature conditions. The evaluation of sinterability in sintered bodies was conducted using the Archimedes method and electron microscopy. The presence of α-Fe phase in the obtained sintered body was evaluated by XRD pattern, and the evaluation of magnetic properties was conducted by VSM. Furthermore, the entire process from sintering aid addition step to sintering was conducted in a low-oxygen atmosphere.
Results
As a result of the exploration of low-melting point compositions of Ba based alloys, we found the alloy with a melting point of 270 ℃ (< 300 ℃) and achieved sintered bodies with a relative density of over 85 % even at sintering temperatures of below 350 ℃. According to the XRD profile, lowering the sintering temperature suppressed the precipitation of the α-Fe phase, and the coercivity did not decrease.
References
[1] R. Matsunami, M. Matsuura, N. Tezuka, S. Sugimoto, J. Magn. Soc. Jpn., 44, 64-69 (2020).
[2] Niterra Co., Ltd., AIST, press release “Development of high-density technology for Sm2Fe17N3 permanent magnet” (10.9.2024).
[3] Y. Hirayama, A. K. Panda, T. Ohkubo, K. Hono, Scr. Mater., 120, 27-30 (2016).
[4] A. Hosokawa, W. Yamaguchi, K-Suzuki, K. Takagi, J. Alloys Compds., 869, 159288 (2021).